Excitatory amino acid receptors: a gallery of new targets for pharmacological intervention.

The excitatory amino acids (EAAs) L-glutamate and L-aspartate are the most abundant amino acids in brain and play a number of roles in maintaining neuronal function. Among these are their use as protein constituents, as key intermediates in ammonia metabolism, and as precursors for other neurotransmitters. Given the widespread distribution of EAA-containing neurons, these transmitters are likely to be involved in virtually all central nervous system functions, with abnormalities in neurotransmission contributing to the symptoms of a host of neurological and psychiatric disorders. Because of the importance of EAAs in maintaining the functional integrity of the central nervous system, efforts are underway to design agents capable of regulating the activity of these transmitters for therapeutic gain. Inasmuch as potential side effects preclude a generalized modification of this system, strategies must be found to alter EAA neurotransmission in selected brain regions. In this regard, pharmacological data suggest several functionally distinct EAA receptors, a finding confirmed by cloning studies which hint at an even larger family of sites. Moreover, it appears that some excitatory amino acid receptor complexes are composed of interacting sites which orchestrate receptor function, and there is evidence that EAA receptors may influence the activity of one another. Thus, there appear to be numerous sites that can be targeted to selectively modify excitatory amino acid neurotransmission in brain. Besides the agonist recognition site for each receptor subtype, other targets include regulatory subunits, ion channels and components of receptor-coupled second messenger systems.

Examination of the D2/5-HT2 affinity ratios of resolved 5,6,7,8,9,10-hexahydro-7,10-iminocyclohept[b]indoles: an enantioselective approach toward the design of potential atypical antipsychotics.

Enantiomers of several N-substituted 5,6,7,8,9,10-hexahydro-7,10-iminocyclohept[b]indoles were obtained by the resolution of 2-fluoro-5,6,7,8,9,10-hexahydro-7,10-iminocyclohept[b]indole and 5,6,7,8,9,10-hexahydro-7,10-iminocyclohept[b]indole followed by N-alkylation. These, as well as the racemates, were evaluated for their affinity for the 5-HT2 and D2 receptors. Those compounds possessing the 7S,10R stereochemistry were consistently recognized by the 5-HT2 and D2 receptors as the eutomer. 2-Fluoro-11-[4-(4-fluorophenyl)-4-oxobutyl]-5,6,7,8,9,10-hexahydro-7S,10 R- iminocyclohept[b]indole [(7S,10R)-8] had the highest affinity for the 5-HT2 receptor (Ki = 0.80 nM), while its distomer (7R,10S)-8 was the most selective member of this class of bridged gamma-carbolines (D2/5-HT2 = 562). Incorporation of a benzoyl or isosteric benzisoxazole moiety tethered by a four-carbon spacer to a bridged gamma-carboline nucleus, possessing the 7S,10R absolute configuration, produced high affinity ligands for the 5-HT2 and D2 receptors.

Behavioral pharmacology of NPC 17742, a competitive N-methyl-D-aspartate (NMDA) antagonist.

The behavioral effects of the competitive N-methyl-D-aspartate (NMDA) antagonist 2R,4R,5S-2-amino-4,5-(1,2-cyclohexyl)-7-phosphonoheptanoate (NPC 17742) were compared with those of its parent compound, 2-amino-4,5-(1,2-cyclohexyl)-7-phosphonoheptanoate (NPC 12626), and other reference agents in a variety of operant-based tasks in rodents. In mice trained to lever press under a fixed-ratio (FR) 20 reinforcement schedule, NPC 17742 was 6.2 times more potent than NPC 12626 and equipotent with the competitive NMDA antagonist [E]-2-amino-4-methyl-5-phosphono-3-penteneoic acid (CGP 37849) in reducing rates of responding. NPC 17742 was also 3.5 and 4.5 times more potent than [+-]cis-4-phosphonomethyl-2-piperidine carboxylate (CGS 19755) and [+-] 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP), respectively, and half as potent as 3SR, 4aRS, 6SR, 8aRS-6-(phosphonomethyl)-1,2,3,4,4a,5,6,7,8,8a- decahydroisoquinoline-3-carboxylate (LY 274614) in this paradigm. In rats trained to discriminate 4.0 mg/kg NPC 17742 from saline, NPC 17742 was 5.7 times more potent than NPC 12626 in substituting for NPC 17742. CGS 19755 also substituted for NPC 17742, but a maximum of only 50% NPC 17742 lever responding was observed after LY 274614 administration. In rats trained to lever press in a modified Geller-Seifter procedure, NPC 17742 and NPC 12626, like the benzodiazepine chlordiazepoxide, increased rates of punished responding. Neither tolerance nor sensitization to the anti-punishment effects were observed upon administration of NPC 17742 for 5 consecutive days. The results are consistent with NPC 17742 being a potent, systemically active compound whose behavioral effects are mediated through interaction with the NMDA receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

The envelope glycoprotein of HIV-1 alters NMDA receptor function.

Human immunodeficiency virus (HIV-1) infection often results in central nervous system (CNS) dysfunction, yet the mechanism(s) of action for HIV-1 in the CNS are not fully understood. In the present study gp120, the HIV-1 envelope glycoprotein, was shown to selectively inhibit N-methyl-D-aspartate (NMDA) receptor function. In addition to inhibiting radioligand binding to rat NMDA receptors, gp120 inhibited NMDA-induced currents in Xenopus oocytes, attenuated NMDA-stimulated calcium flux and cytotoxicity in cultured cerebellar granule cells, and provided partial protection against NMDA-induced lethality in vivo. These findings suggest that NMDA receptor complex is a possible site of action of HIV-1 within the CNS.

Pharmacological profile of NPC 17742 [2R,4R,5S-(2-amino-4,5-(1, 2-cyclohexyl)-7-phosphonoheptanoic acid)], a potent, selective and competitive N-methyl-D-aspartate receptor antagonist.

2R,4R,5S-(2-amino-4,5-(1,2-cyclohexyl)-7-phosphonoheptanoic acid) (NPC 17742), the most potent isomer of the mixture 2-amino-4,5-(1,2-cyclohexyl)-7-phosphonoheptanoic acid (NPC 12626), was evaluated for activity in tests associated with receptors for excitatory amino acids. In receptor binding assays, NPC 17742 was selective for the N-methyl-D-aspartate (NMDA) receptor with a potency comparable to that of D(-, -3-(2-carboxypiperazine-4-yl)propyl-1-phosphonic acid. Like (+/-)cis-4-phosphono-methyl-2-piperidine carboxylic acid (CGS 19755) and (+/-)(E)-2-amino-4-methyl-5-phosphono-3-penteneoic acid (CGP 37849), NPC 17742 competitively inhibited NMDA-induced enhancement of 1-[(2-thienyl)cyclohexyl]piperidine binding to the NMDA receptor ionophore and partially inhibited [3H]glycine binding to strychnine-insensitive sites. In contrast, NPC 17742 and CGP 37849 inhibited Mg(++)-stimulated 1-[(2-thienyl)cyclohexyl]piperidine binding in a noncompetitive fashion. In voltage-clamped Xenopus oocytes expressing excitatory amino acid receptors, NPC 17742 (pKB = 6.91) was equipotent with CGP 37849 (pKB = 7.17) in inhibiting NMDA-induced inward currents. Likewise, NPC 17742 (ED50 = 2.68 mg/kg) was equipotent with CGP 37849 and CGS 19755 in blocking NMDA-induced convulsions, but was less potent than these two compounds in the maximal electroshock test. Unlike CGP 37849 or CGS 19755, NPC 17742 potently antagonized seizures induced by pentylenetetrazol. In a model of global ischemia, low doses of NPC 17742 given either before or after ischemic result were effective in blocking damage to hippocampal CA1 neurons. The pharmacologic responses to NPC 17742 occurred at doses 30- to 300-fold lower than the acute lethal dose.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of strychnine-insensitive glycine receptor antagonists and sigma agents on working memory performance: comparison with dizocilpine and scopolamine.

The strychnine insensitive glycine receptor antagonists (+/-) HA 966 (2.5, 3.5, 4.25 and 5.0mg/kg) and 7 chlorokynurenic acid (5.0, 10.0, and 15.0mg/kg), the putative sigma agents NPC 16377 (5.0 and 8.0mg/kg), BMY 14802 (5.0, 7.5 and 10.0mg/kg), and ifenprodil (5.0 and 7.0mg/kg) and the reference agents scopolamine and dizocilpine [(+) MK 801] were evaluated in a nonspatial delayed matching to sample working memory task in rats. (+/-) HA 966 impaired accuracy at the longest retention interval and decreased response probability measures. 7-Chlorokynurenic acid was essentially without effect. The noncompetitive NMDA antagonist dizocilpine reduced accuracy at all retention intervals, decreased the probability of a choice response and increased the probability of an intertrial interval response. The anticholinergic agent scopolamine selectively reduced accuracy at the longest retention interval but did not affect other performance measures. Sigma agents decreased response probability measures but did not affect accuracy at any retention interval. The results support the notion that sigma agents, glycine antagonists and NMDA antagonists produce different effects in cognitive tasks including working memory performance.

Biochemical and behavioral studies following subchronic administration of GABA uptake inhibitors in mice.

N-(4,4-Diphenyl-3-butenyl) nipecotic acid (SKF(R)-89976A) and N-(4,4-diphenyl-3-butenyl) guvacine (SKF 100330A) are potent inhibitors of the uptake of GABA and have anticonvulsant properties. In the present study, the effects of these compounds on several behavioral and biochemical measures were determined, following subchronic administration. Administration of SKF(R)-89976A (8.9 mg/kg) for 14 days caused a small but significant reduction in its potency to protect against pentylenetetrazole-induced seizures, whereas treatment with SKF 100330A (13.6 mg/kg) had no significant effect. The percentage of animals rendered cataleptic by administration of either GABA uptake inhibitor was reduced by treatment for as few as 4 days and treatment with SKF(R)-89976A for 14 days resulted in a 4-fold increase in the CD50 for induction of catalepsy. The binding of [3H]GABAA and [3H]GABAB in membranes from the forebrain of the mouse were not influenced by treatment with drug nor was synaptosomal uptake of [3H]GABA. Likewise, the binding of [3H]sulpiride in striatal membranes of the mouse was unaffected by repeated exposures to SKF(R)-89976A. These results demonstrate that prolonged administration of GABA uptake inhibitors produced only a small reduction in anticonvulsant potency, whereas liability to side-effects, as demonstrated by the reduction in catalepsy, was substantially reduced.

N-methyl-D-aspartate antagonists and working memory performance: comparison with the effects of scopolamine, propranolol, diazepam, and phenylisopropyladenosine.

The effects of the competitive N-methyl-D-aspartate (NMDA) antagonists CPP (5 & 10 mg/kg) and NPC 12626 (25 & 40 mg/kg) and the noncompetitive NMDA antagonists phencyclidine (1, 3, & 6.25 mg/kg) and MK 801 (0.1 & 0.2 mg/kg) on performance of rats on a nonspatial delayed matching-to-sample working memory task were evaluated. At the highest dose, each NMDA antagonist reduced choice accuracy at all retention intervals. In contrast, the reference anticholinergic agent scopolamine selectively reduced accuracy at long retention intervals, suggesting that scopolamine but not the NMDA antagonists directly interfered with time-dependent working memory retention. Propranolol, diazepam, and phenylisopropyladenosine had little or no effect on choice accuracy, suggesting that noradrenergic, gamma-aminobutyric acid-diazepam, and adenosine receptors may be relatively unimportant for working memory performance as assessed in this task. The NMDA antagonists also differed from scopolamine in that doses of NMDA antagonists that reduced response accuracy also reduced response probability, altered bias (competitive antagonists only), and increased intertrial interval responding (noncompetitive antagonists only). It was concluded that NMDA antagonists disrupt cognitive functions including, but not limited to, those required for accurate working memory performance.

Possible cerebroprotective and in vivo NMDA antagonist activities of sigma agents.

The recent finding that ifenprodil binds with high affinity to sigma sites suggests that other sigma agents may have ifenprodil-like cerebroprotectant and functional N-methyl-D-aspartate (NMDA) antagonist effects. The present study, compared the in vivo effects of ifenprodil and the sigma agents, BMY 14802, caramiphen and haloperidol, in three tests sensitive to NMDA antagonists and purported cerebroprotectant drugs. When administered at or below the rotorod TD50 dose, all four compounds significantly increased survival time in an hypoxic environment (4% O2 in nitrogen). Caramiphen and ifenprodil (ED50 = 52 and 61 mg/kg, respectively) also blocked maximal electroshock-induced seizures, whereas BMY 14802 and haloperidol were ineffective. Finally, caramiphen (ED50 = 95 mg/kg) antagonized seizures and lethality induced by administration of NMDA (250 mg/kg, IP). BMY 14802, haloperidol and ifenprodil only partially antagonized NMDA-induced seizures, but did enhance the anticonvulsant potency of the noncompetitive NMDA antagonist, MK-801. Together, these findings suggest that sigma agents may have cerebroprotective effects.

Identification of phenobarbital N-glucosides as urinary metabolites of phenobarbital in mice.

Previously, the N-glucosylation of phenobarbital had been observed only in humans. The results of a species screen (mouse, rat, guinea pig, rabbit, cat, dog, pig, and monkey) found that only mice excreted the N-glucosides of phenobarbital in urine after ip administration of sodium phenobarbital. The major diastereomer excreted by the mouse had the R configuration at the C-5 position of the barbiturate ring. The N-glucoside metabolites accounted for a small percentage of the dose (approximately 0.5%). Following ip dosing of the mouse with the phenobarbital N-glucosides, free phenobarbital could be detected in the urine. Upon ip or intercerebroventricular (icv) injection of the phenobarbital N-glucosides, minimal CNS activity was observed in the mouse.

Novel class of amino acid antagonists at non-N-methyl-D-aspartic acid excitatory amino acid receptors. Synthesis, in vitro and in vivo pharmacology, and neuroprotection.

The isoxazole amino acid 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl) propionic acid (AMPA) (1), which is a highly selective agonist at the AMPA subtype of excitatory amino acid (EAA) receptors, has been used as a lead for the development of two novel EAA receptor antagonists. One of the compounds, 2-amino-3-[3-(carboxymethoxy)-5-methylisoxazol-4-yl]propionic acid (AMOA, 7), was synthesized via O-alkylation by ethyl chloroacetate of the amino acid protected AMPA derivative 4. The other compound, 2-amino-3-[2-(3-hydroxy-5-methylisoxazol-4-yl)-methyl-5-methyl-3-+ ++oxoisoxazolin -4-yl]propionic acid (AMNH, 14) was synthesized with use of 4-(chloromethyl)-3-methoxy-5-methylisoxazole (8) as the starting material. The intermediate 4-(chloromethyl)-2-(3-methoxy-5-methylisoxazol-4-yl)methyl-5-me thylisoxazolin- 3-one (11) was converted into the acetamidomalonate (12), which was stepwise deprotected to give 14. Compounds 7 and 14 were stable in aqueous solution at pH values close to physiological pH. Neither 7 nor 14 showed detectable affinities for the receptor, ion channel, or modulatory sites of the N-methyl-D-aspartic acid (NMDA) receptor complex. Quantitative receptor autoradiographic and conventional binding techniques were used to study the affinities of 7 and 14 for non-NMDA receptor sites. Both compounds were inhibitors of the binding of [3H]AMPA (IC50 = 90 and 29 microM, respectively). Compounds 14 and 7 were both very weak inhibitors of the high-affinity binding of radioactive kainic acid [( 3H]KAIN). Compound 14, but not 7, was, however, shown to be an inhibitor of low-affinity [3H]KAIN binding (IC50 = 40 microM) as determined in the presence of 100 mM calcium chloride. In the rat cortical slice preparation, 7 was shown to antagonize excitation induced by 1 with some selectivity, whereas 14 proved to be a rather selective antagonist of KAIN-induced excitation. Both antagonists showed very weak effects on the excitatory effects of NMDA. Compound 7 was a poor antagonist of excitation by quisqualic acid (2), whereas 14 did not affect excitation by this nonselective AMPA receptor agonist. On cat spinal neurones, both 7 and 14 reduced excitations by 1 and KAIN, but, again, the excitatory effects of 2 were much less sensitive. Compound 14 and, in particular, 7 effectively protected rat striatal neurones against the neurotoxic effects of KAIN, whereas the toxic effects of 1 were reduced only by 7. Neither antagonist showed protection against the cell damage caused by intrastriatal injection of the NMDA agonist quinolinic acid.(ABSTRACT TRUNCATED AT 400 WORDS)

NMDA receptor agonists derived from ibotenic acid. Preparation, neuroexcitation and neurotoxicity.

The two heterocyclic aspartic acid and glutamic acid analogues derived from ibotenic acid, (RS)-2-amino-2-(3-hydroxy-5-methylisoxazol-4-yl)acetic acid (AMAA) and (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) have previously been shown to be selective agonists at N-methyl-D-aspartic acid (NMDA) and AMPA receptors, respectively. Two analogous series of AMAA and AMPA derivatives have now been synthesized and characterized in receptor binding studies and neuropharmacological experiments. AMAA was shown to be a very potent NMDA agonist in cortical tissue preparations, slightly more active than NMDA, whereas N-methyl-AMAA was less potent and N,N-dimethyl-AMAA almost inactive. (RS)-3-Hydroxy-4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridine-4-carboxylic acid (4-HPCA), a bicyclic analogue of AMAA, exhibited weak NMDA agonist effects similar to those of quinolinic acid. The relative potency as AMPA receptor agonists of AMPA, N-methyl-AMPA, N,N-dimethyl-AMPA and (RS)-3-hydroxy-4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridine-5-carboxylic acid (5-HPCA), a bicyclic analogue of AMPA, was distinctly different from that of the AMAA series of compounds as NMDA agonists. The pharmacological and toxicological profiles of AMAA and 4-HPCA, compared with those of quinolinic acid, are consistent with heterogeneity of NMDA receptors.

Pharmacologic profile of NPC 168 (naltrexone phenyl oxime), a novel compound with activity at opioid receptors.

NPC 168 (naltrexone phenyl oxime) was synthesized as a novel opioid antagonist and evaluated in several in vitro and in vivo assays. NPC 168 inhibited binding to the mu, delta and kappa subtypes of the opioid receptor with nanomolar potencies. The potency of NPC 168 to antagonize morphine-induced analgesia was slightly less than that of naltrexone and nalmefene following either intraperitoneal (ED50 = 0.07 mg/kg) or oral (ED50 = 0.82 mg/kg) administration. The duration of action of NPC 168 was approximately 8 hr following subcutaneous administration, compared to 4 hr for nalmefene, to antagonize oxymorphonazine-induced analgesia. The long duration of action of NPC 168 was substantiated by pharmacokinetic data that demonstrated rapid uptake and slow clearance of NPC 168 from brain. NPC 168 (5, 10 and 20 mg/kg) also inhibited cumulative 6-hr food intake in rats that were deprived of food for 24 hr, but chronic administration of this compound to rats over a three-week period resulted in a marginal reduction in cumulative body weight gain. NPC 168 at doses of up to 10 mg/kg did not produce a conditioned taste aversion. However, NPC 168 was slightly more toxic than either naltrexone or nalmefene when administered parenterally, and as toxic as nalmefene when administered by the oral route. These data demonstrate that NPC 168 is a novel opioid antagonist with a longer duration of action than either naltrexone or nalmefene.

Evidence for direct interactions between the NMDA and glycine recognition sites in brain.

The interaction between glycine and competitive N-methyl-D-aspartate (NMDA) antagonists was investigated. Glycine (IC50 = 170 nM) partially (approximately 60%) inhibited [3H]CGS-19755 ((+/-)-4-phosphonomethyl-2-piperdine carboxylic acid), but not [3H]CPP (3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid) binding. The action of glycine was mimicked by D-serine and antagonized by 7-chlorokynurenate. CGS-19755 (IC50 = 230 nM) partially inhibited [3H]glycine binding from strychnine-insensitive sites; this effect was antagonized by NMDA. CPP and NPC 12626 (2-amino-4,5-(1,2-cyclohexyl)-7-phosphonoheptanoic acid) inhibited [3H]glycine binding, but only at concentrations 100- to 1000-fold greater than required to displace [3H]CGS-19755 or [3H]CPP. These data provide the first evidence for bidirectional interactions between glycine and NMDA recognition sites and suggest pharmacological differences among competitive NMDA antagonists.

Footshock-induced freezing behavior in rats as a model for assessing anxiolytics.

A number of chemically distinct anxiolytics were examined for effects on defensive behavior (foot-shock-induced freezing) in rats. Central nervous system acting drugs which are not anxiolytics were also studied. Animals were injected with a drug or vehicle (IP) prior to being placed in a chamber with a grid floor through which two footshocks were delivered. Behavior was observed during the pre-shock period (2 min) and for 4 min after the second footshock. The effects of the following drugs on the duration of footshock-induced freezing were studied: diazepam (DZP); 2-amino-4,5-(1,2-cyclohexyl)-7 phosphonoheptonic acid (NPC 12626); 3-((+/-)-2-carboxypiperazine-4-yl)-propyl-l-phosphonic acid (CPP); [(+)-5-methyl-10-11,dihydroxy-5H-dibenzo(a,d)cyclohepten-5,10- imine (MK-801); buspirone hydrochloride (BUS); DL-amphetamine sulfate (AMP); haloperidol (HAL); ethyl-beta-carboline-3 carboxylate (beta-CCE). Compounds which reduced the duration of footshock-induced freezing included DZP, BUS, and the competitive NMDA antagonists NPC 12626 and CPP. The non-competitive NMDA antagonist, MK-801, had no effect on the response. The highest dose of amphetamine tested also reduced footshock-induced freezing. However, amphetamine-treated animals did not locomote or rear after footshock, suggesting fear of the environment. Animals injected with DZP, NPC 12626, CPP or buspirone spent at least 1.4 of the 4 post shock minutes locomoting. Haloperidol had no effect on freezing at the doses tested. beta-CCE tended to increase the duration of footshock-induced freezing.(ABSTRACT TRUNCATED AT 250 WORDS)

gamma-Aminobutyric acidB receptor activation modifies agonist binding to beta-adrenergic receptors in rat brain cerebral cortex.

The interaction of isoproterenol with beta-adrenergic receptor (beta AR) binding sites was measured in membranes prepared from rat brain cerebral cortical slices previously incubated in the presence or absence of gamma-aminobutyric acid (GABA) receptor agonists. Both GABA and baclofen, but not isoguvacine, altered beta AR agonist binding by increasing the affinity of both the low- and high-affinity binding sites and by increasing the proportion of low-affinity receptors. The response to baclofen was stereoselective, and the effect of GABA was not inhibited by bicuculline. The results suggest that GABAB, but not GABAA, receptor activation modifies the coupling between beta AR and stimulatory guanine nucleotide-binding protein, which may in part explain the ability of baclofen to augment isoproterenol-stimulated cyclic AMP accumulation in brain slices.

Pharmacological profile of NPC 12626, a novel, competitive N-methyl-D-aspartate receptor antagonist.

The novel compound 2-amino-4,5-(1,2-cyclohexyl)-7-phosphonoheptanoic acid (NPC 12626) was evaluated for activity in a variety of tests associated with receptors for excitatory amino acids. NPC 12626 failed to inhibit the specific binding of RS-[3H] amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid or [3H] kainic acid to brain membranes in vitro but displaced both agonist and antagonist binding to N-methyl-D-aspartic acid (NMDA) receptors. Like cis-(+/-)-3-(2-carboxypiperazine-4-yl)propyl-1-phosphonic acid, NPC 12626 competitively blocked NMDA-induced enhancement of [3H]-1-thienylcyclohexyl)piperidine binding. In the voltage-clamped frog oocyte expression system, NPC 12626 was a competitive inhibitor of NMDA-evoked inward current with a pA2 of 6.24. After both i.c.v. or i.p. administration, NPC 12626 was a potent anticonvulsant in the pentylenetetrazol, maximal electroshock and NMDA seizure models. Furthermore, low doses (25 mg/kg) of NPC 12626 given i.v. were effective in preventing damage to the CA1 region of hippocampus in the gerbil model of global ischemia. Unlike the noncompetitive NMDA antagonist, phencyclidine, but like cis-(+/-)-3-(2-carboxypiperazine-4-yl)propyl-1-phosphonic acid and pentobarbital, NPC 12626 only partially substituted for phencyclidine in a drug discrimination study. The results of the current study indicate that NPC 12626 is a novel, systemically active and competitive NMDA receptor antagonist.